High-pressure sodium lamps are well-known in the lighting field, and are currently in wide use by many city utilities for street lighting purposes. As a person skilled in the art would know, although such lamps have a long lifespan, they eventually fail over time, in part because of electrode depletion and deposition of the electrode material on the interior of the arc tube. This deposition results in heat retention, and, as the darkening of the arc tube increases, a point is reached where lamp voltage can no longer maintain a continuous arc. The result is a cycling condition in which the lamp continually flashes or attempts to start. The cycling is not always easy to detect and correct in a quick and cost-effective manner. Further, cycling can be visually distracting. It can also be annoying, especially in residential areas, as it can result in radio and television interference.
Conventional high pressure sodium lamps are typically photocell controlled when used in conjunction with street light installations. The photocell control, or in some cases, a timeclock, either enables power supply to the lamp, or cuts it off, depending on whether it is night or day. Power is typically supplied to the lamp by a pair of conventional electrically conductive wires or leads, and the photocell control is positioned in series in one of such leads.
FIG. 1, which is labeled “prior art,” schematically illustrates the circuitry of a conventional, photosensor-controlled, high-pressure sodium lamp (without an anti-cycling control). Each lamp is normally powered by a line voltage which may be, for example, of 120 volts AC, provided by lines 1, 3. A photocell sensor control 5, positioned in series between the power source and the lamp, is used to differentiate between day and night, so that power can be supplied to the lamp at dusk and power can be discontinued at dawn.
In the evening, when the photosensor control 5 initially causes power to be supplied, the lamp is initially in an unlit condition. Such lamps have a ballast choke/transformer 7 with a secondary winding or coil 9 that is connected to a pulsing starter device 11. When power is initially supplied, the starter device 11 sends pulses to the secondary coil 9. This causes the ballast 7 to act as a step-up transformer that generates high voltage spikes of several thousand volts across the lamp's electrodes 13, 15, and consequently results in ignition of the lamp. Once ignition occurs, current flow through the ballast causes the lamp voltage to drop (typically from about 150 to 55 volts AC), and pulsing from the starter device 11 ends. If the lamp cannot hold ignition, the lamp will repeatedly attempt to restart. The lamp may restart and remain lit once it has cooled sufficiently to allow sodium ionization to once again take place.
Obviously, cycling is correctable by simply replacing a depleted lamp. However, if a cycling condition is allowed to continue over a period of time, it eventually damages the lamp's starter/ballast unit 7, 11, commonly by burning out the ballast 11. When this happens, the lamp ceases to cycle, but the starter/ballast unit must then be replaced along with the depleted lamp, resulting in higher overall costs of repair. For such reason, it is important to detect a cycling condition as soon as possible. In addition, the attempt to restart a cycling lamp may result in substantial radio frequency interference as the starter pulses the ballast with high voltage pulses.
From the standpoint of labor, many or most city utilities have no cost-effective means for quickly detecting when such lamps are cycling. The typical utility does not have large numbers of service personnel constantly checking street lamps at night, which is the only time cycling is apparent since such lamps normally do not operate during the day. As a result, cycling may continue for extended periods.
Furthermore, cycling is difficult to detect even in situations where service checks are made at night. Depending on the level of arc tube darkening, a cycling lamp may remain lit for several minutes or more before it loses its arc and attempts to restart. This may require a service person to visually monitor individual lamps for more than just a brief period of time in order to discover whether cycling is occurring.
Since high-pressure sodium lamps have a predicted service life, most city utilities have simply taken to automatically replacing groups of lamps at selected times after they have been placed in service, regardless of whether or not a significant number of such lamps have actually begun to cycle. This is inefficient because it too often results in an earlier than necessary lamp replacement, or replacement after many lamp ballasts have already been injured from cycling incidents, and consequently, does not make optimum use of each lamp.
Historically, high-pressure sodium lamps went into large-scale result of the energy shortages created by an Arab oil embargo in or about that time. High-pressure sodium lamps have approximately twice the energy efficiency of their predecessors, mercury vapor lamps, which were the most common street lamps in use before that time. The above-described cycling problem continues to be pressing, and must be solved in a way that will maximize the life of existing lamps in an easy-to-implement, cost-effective manner.
The patent literature discloses that few inventors or companies have yet had occasion to address the above problem. One notable exception involves the efforts of Area Lighting Research, a Hackettstown, N.J. company. Area Lighting is the assignee of two U.S. patents, one issued on Jun. 10, 1980 to Duve et al. (U.S. Pat. No. 4,207,500), and the other issued on Sep. 25, 1984 to Lindner et al. (U.S. Pat. No. 4,473,779). Both patents specifically relate to the cycling malfunction of high-pressure sodium lamps, and each offers a solution, albeit one that is different from the invention disclosed here. It should be mentioned in passing that both patents provide a much more detailed description of the cause of the cycling malfunction than the cursory explanation provided above.
Duve et al. discloses a cut-off device that activates a relay in response to a signal from a detector-signal generator that senses when the voltage increase across the lamp is greater in magnitude than the lamp's normal operating voltage. The increase in voltage corresponds to the lamp's attempt to relight itself. A timing circuit monitors the signal from the detector-signal generator, and determines whether the sensed increase in voltage constitutes undesirable cycling. If so, the timing circuit activates the relay, thus cutting off power to the lamp.
Lindner et al. claims to be an improvement over Duve, and determines cycling by sensing a change in lamp power factor. In doing so, Lindner uses the combination of both a voltage signal generator and a current signal generator which simultaneously transmit their signals to a comparator-processor, where the latter compares their phases. When their phases have a certain known relationship that corresponds to cycling, Lindner similarly activates a relay cutting off power to the lamp.
U.S. Pat. No. 5,103,137 to Blake, et al. discloses an anti-cycling device for high pressure sodium lamps that uses changes in lamp current to monitor lamp cycling and, after a certain number of cycling events, cuts off power to the lamp. The anti-cycling device has a current sensor connected in series to one lead between the photocell control and the lamp. Such sensor is operative to develop a continuous AC voltage signal that is generally proportional to the magnitude of the alternating current in the lead as current passes through the lamp. An extinguished lamp that either initially starts in the beginning of an evening, or attempts to restart as a result of cycling, draws higher than normal current levels. This, in turn, creates a higher than normal alternating voltage output from the sensor. In view of its teachings concerning the use of anti-cycling devices for high pressure sodium lamps, this patent is incorporated by reference herein.
In the device of this patent, a first amplifier is connected to the current sensor in a manner so that it continuously senses the sensor's voltage output, and generates an amplified AC voltage output signal whose magnitude is also generally proportional to the sensor voltage. This output is rectified by a set-point diode, and is transmitted to another amplifier. The second amplifier receives such signal and compares its magnitude to the level of a preselected threshold signal. The latter amplifier, in response to the first amplifier's output, is operative to output a trigger signal every time the first amplifier's rectified output exceeds the threshold level. A counter receives and counts each trigger signal transmitted from the second amplifier. It is programmed to output a malfunction or cut-off signal in the event it counts a certain preselected number of trigger signal transmissions (such as three) during a given time period. Once the preselected number of trigger signals is reached, a relay is activated to interrupt power to the lamp until the counter is reset. An LED may be turned on to indicate that the lamp is cycling and needs to be replaced. The device may be reset when the power to the lamp is turned off and then back on, as when the photocell sensor interrupts power upon detecting a daylight condition and then restores power when darkness is once again detected.
U.S. Pat. No. 6,028,396 to Morrissey, Jr., et al. discloses a system that includes detector circuitry for detecting the load drawn by the lamp and a microcontroller programmed to predict lamp condition, such as cycling and lamp-out conditions based on the detected load. The circuitry can shut the lamp off if a cycling condition is detected. A visual indication of the detected condition may be outputted by the circuitry.
As will become apparent, the present invention provides an anti-cycling device that is simpler in both design and operation than any of the devices discussed above. Further, the device disclosed here is low in cost, extremely reliable, and is equally well-suited for either retrofitting to street lamps presently in use, or factory installation by the lamp manufacturer.